This invention relates to improvements in electric power assisted steering apparatus and in particular to gearbox assemblies for use in electric power assisted steering systems of the kind which incorporate a worm and wheel gear assembly for transferring torque from an electric motor to a steering shaft or output shaft operatively connected thereto.
It is known to provide a power steering system for a vehicle comprising an electric motor having a stator and a rotor, an input shaft operatively connected to the rotor and adapted to rotate therewith, an output shaft associated with a steering shaft, and a gearbox adapted to transfer torque from the input shaft to the output shaft in response to a measure of the torque in the output shaft as produced by a torque sensor. The motor is typically operated to apply an increasing torque to the output shaft as the measured torque increases, thus applying assistance that helps reduce the effort needed to steer the vehicle.
In a simple arrangement the input shaft carries a worm gear, and the output shaft carries a wheel gear. The teeth of the worm and wheel intermesh to transfer the torque. This system is simple and robust whilst providing relatively high gearing with a low component count. There exists, however, a problem in such gearboxes with noise and vibration due to incorrect meshing between the worm and wheel.
A worm and wheel gearbox, for an electric power steering system, having a fixed centre distance between the axes of the worm and gear wheel is prone to rattle when transmitting zero or low torque if there is excessive clearance between the gear teeth of the worm and those of the gear wheel. Alternatively, it is prone to having high friction if there is forced meshing between the worm and wheel due to interference between the teeth.